When working on posterior teeth, a dentist generally uses a metal matrix retainer band which measures approximately a thousandth of an inch in thickness and is approximately eight millimeters wide. In order to place the band around a tooth, a holding mechanism is required. This mechanism is commonly known as a matrix retainer clamp and typically consists of an end into which the ends of the looped matrix band are inserted and tightened into place, as well as a shaft through which a sliding tightening screw allows for adjustment of the matrix band. At the same time, the shaft provides the dentist with a way of holding the matrix band for placement.
Once in place around the tooth and tightened, the band, due to curvature or angulation of the head of the retainer clamp, pulls tightly around the gingival margin (area of the cavity prep that approximates the gum tissue). Due to the anatomical shape of many teeth the interproximal wall becomes concave, thereby leaving an opening through which filling material would push upon condensation. Placement of a wooden triangular wedge between the adjacent tooth and the matrix band is required not only to tighten the band to the tooth but also to spread the teeth slightly apart so that upon matrix band removal the teeth will still make contact proximally.
In North America, the most common matrix retainer clamp is the Toffelmire Matrix Retainer. The retainer clamp is made of stainless steel, with three choices in the head for placement of the band: (1) left side upper or right side lower, (2) right side upper or left side lower, or (3) straight. The shaft contains a long tightening screw with exposed threads. Thoughtful care must be exercised to avoid binding the lip tissue into the screw during the tightening procedure. Furthermore, due to the exposure of the thread mechanism, continuous lubrication is required to keep the action moving freely.
During the wedging procedure, it is common to access the proximal area from the palatal or lingual (tongue) side. In situations where a tooth has an anomalous shape or position, however, it is sometimes necessary to wedge from the vestibular (cheek) side. This is not always possible with the Toffelmire Matrix as the retainer head usually blocks the desired passage to the interproximal area.
Although it is possible to cut the matrix band for removal from the tooth by pulling the band laterally, in practical terms the band is loosened and slipped off the tooth in the reverse of the placement procedure and following wedge removal. Since the Toffelmire Matrix Retainer has only one vertical stop the band can twist out of the head and become difficult to remove without damaging the filling.
Most dental offices work with a tray set up for each type of procedure. It would be necessary to have at least two Toffelmire Matrix Retainers on each filling tray to allow for location choice and sterilization. In practical terms, it is apparent that clinicians often have only two retainers per operatory and use a cold sterilization technique, usually with a new band for each patient. With the advent of highly infective diseases such as Hepatitis B and AIDS, along with the placement of the matrix band in an area that bleeds readily, it would be prudent to have a band retainer mechanism which eliminates any possibility of cross contamination, i.e., one which is disposable as a single entity.